Thermoplastic elastomers are an interesting class of materials which bridge the gap between conventional rubber and traditional thermoplastics. Thermoplastic elastomers have properties similar to vulcanized rubber. However, unlike vulcanized rubber they will soften or melt when heated as is characteristic of thermoplastics. Therefore, reprocessing of scrap or unusable components is possible. In addition, thermoplastic elastomers can undergo thermoplastic molding operations such as injection and blow molding. They are more readily extrudable than vulcanizable rubber. Also, they can be utilized in vacuum forming processes or as hot melt adhesives, neither of which is possible with conventional rubber.
The potential of polyisobutylene block copolymers for use as thermoplastic elastomers is well recognized. The properties of the materials are determined by the choice of the monomer for the hard block, the relative molecular weights of the hard and soft segments, and the choice of overall molecular architecture, e.g. linear structure versus a star structure. Despite the many possible structural variations, the choice of monomers compatible with these synthetic methodologies does not facilitate easy access to materials which exhibit hydrophilic character, crosslinking potential, and many other useful chemical functions in addition to rubbery properties. Several methods, as discussed below, have been disclosed for their preparation.
Traditionally, synthesis of thermoplastic elastomers generally involved preparation of diblock copolymers having a rubbery segment and a plastic segment at least one of which remains nonterminated and reactive. Subsequently, a di- or multifunctional compound is added to the non-terminated diblocks combining at least two of the blocks in a head-to-head orientation to produce a linear triblock composition. The product generally comprises a plastic segment connected to a rubbery segment which is, in turn, connected to a plastic segment. The plastic segment is usually a vinyl aromatic or A block, such as polystyrene, while the rubbery segment has in the past usually been a diene or B block, such as polybutadiene or polyisoprene; hence the abbreviation for the linear triblock is ABA.
Preparation via known synthesis of diblocks and the subsequent combining thereof involves anionic polymerization systems, generally organolithium initiated, to form ABLi diblocks which are linked together with a difunctional compound susceptible to carbanion attack such as diisocyanates, divinylbenzene, dibromoethane, epoxidized linseed oil or silicon tetrachloride. Examples of such processes are set forth in U.S. Pat. Nos. 3,639,517 and 3,639,521. When a polyfunctional linking compound is employed, such as silicon tetrachloride, a radial or branched structure results wherein four diblocks are linked together. By controlling the amount of selected difunctional linking compounds such as divinylbenzene or diisocynates, a multifunctional nucleus is developed which can also link together a plurality of ABLi diblocks. Such a process is set forth in U.S. Pat. Nos. 3,985,830 and 4,108,945. More recently, other processes, as set forth below, have been reported.
For instance, living catalysts have been used to polymerize olefins. In a living polymerization, each catalyst molecule initiates a growing polymer chain that does not undergo chain transfer or termination reactions while monomer is present. By comparing the number of initiator molecules with the number of polymer chains produced in the final polymer, one can determine whether or not a living polymerization has occurred. These two numbers should be equivalent to be a true living polymerization. If there are a substantially greater number of chains, then the polymerization is not living. (See U.S. Pat. Nos. 5,506,316 and 5,403,803 which are herein incorporated by reference.) For example, U.S. Pat. No. 5,428,111 ("the '111 patent") discloses a process for the production of block copolymers by: (a) contacting cationically-polymerizable monomer with an initiator to produce living polymer; (b) contacting this with a capping compound selected from the group consisting of diphenyl alkylene, alpha-methoxystyrene, trans-stilbene, 1-isopropenyl-naphthalene, and 2,4-dimethyl-alpha-methyl-styrene; and (c) contacting the capped polymer with cationically-polymerizable monomer(s).
The '111 patent also discloses: (i) a method comprising contacting a cationically-polymerizable monomer with a polymer capped with the above compounds; (ii) a block copolymer comprising mid-blocks of isoolefin polymer and end-blocks of aromatic polymers; and (iii) the use of the polymers produced as thermoplastic elastomers. The '111 patent discloses a catalyst composition consisting of a tertiary organic halide, titanium chloride, and a hindered pyridine.
Japanese Pat. Application JO 6287254-A discloses the preparation of block copolymer(s) comprising polymerization of cationically polymerizable vinyl monomer(s) in the presence of a polymerization initiator composed of Lewis acid(s) and compound(s) having a moiety represented by the formula CR.sub.1 R.sub.2 R.sub.3 groups, wherein R.sub.1 =H, alkyl or aryl; R.sub.2 =alkyl or aryl; and R.sub.3 =halogen, alkoxy or acyloxy. The Japanese '254-A patent application abstract discloses a catalyst composition consisting of a tertiary amine, titanium chloride, and an amine.
U.S. Pat. No. 5,451,647 discloses an olefin polymerization process wherein an olefin chargestock is contacted with an organic compound polymerization initiator, a Lewis acid coinitiator and a pyridine compound such as 2,6-di-tert-butylpyridine to produce homopolymers, copolymers or block copolymers having a narrow molecular weight distribution. The '647 patent discloses a catalyst composition consisting of a tertiary organic halide, dimethylaluminum chloride, and a hindered pyridine. Randomly functional polyisobutylenes have been prepared from random, homogenous copolymers of p-methylstyrene ("pMS") and isobutylene using non-living polymerization techniques, subsequent halogenation and functionalization (See U.S. Pat. Nos. 5,430,118; 5,426,167; and 5,162,445 which are incorporated herein by reference). Copolymers with low (&lt;5 mol %) incorporation of pMS are commercially available under the tradename as "XP-50" available from Exxon Chemical Co. U.S. Pat. No. 5,162,445 discloses many ways to functionalize and graft XP-50 into useful rubbery materials. However, the prior art does not explicitly disclose a way to carry out living copolymerization of an isoolefin and pMS followed by sequential addition of styrene based monomers.
Living polymerization of isobutylene and p-methylstyrene has been described by Kennedy in two articles, "Living Carbocationic Copolymerizations. I. Synthesis and Characterization of Isobutylene/p-Methylstyrene Copolymers" (Journal of Physical Organic Chemistry, Vol. 8, pp. 258-272, 1995) and "Living Carbocationic Copolymerizations. II. Reactivity Ratios and Microstructure of Isobutylene/p-Methylstyrene Copolymers" (Journal of Physical Organic Chemistry, Vol. 8, pp. 273-281, 1995). However, these articles do not disclose a way to block polymerize the copolymers. Therefore, a need still exists for a process to produce triblock copolymers having an isoolefin/styrene midblock.